Canister status determination

ABSTRACT

A method and apparatus are disclosed for determining status of a canister of a topical negative pressure (TNP) system. The method includes the steps of monitoring pressure provided by a pump element of the TNP system, determining at least one characteristic associated with the monitored pressure and determining status of at least one parameter associated with a canister of the TNP system responsive to the determined characteristics.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/672,468, filed on Feb. 5, 2010, which is a U.S. National Phase of thePCT International Application No. PCT/GB2008/002346, filed on Jul. 9,2008, which claims priority to Great Britain Patent Application No.0715259.8, filed on Aug. 6, 2007. The disclosure of these priorapplications is incorporated by reference in its entirety and should beconsidered a part of this specification.

BACKGROUND Technical Field

The present invention relates to apparatus and a method for theapplication of topical negative pressure (TNP) therapy to wounds. Inparticular, but not exclusively, the present invention relates to amethod and apparatus for determining status such as a fullnessassociated with a canister in a TNP system.

Description of the Related Art

There is much prior art available relating to the provision of apparatusand methods of use thereof for the application of TNP therapy to woundstogether with other therapeutic processes intended to enhance theeffects of the TNP therapy. Examples of such prior art include thoselisted and briefly described below.

TNP therapy assists in the closure and healing of wounds by reducingtissue oedema; encouraging blood flow and granulation of tissue;removing excess exudates and may reduce bacterial load and thus,infection to the wound. Furthermore, TNP therapy permits less outsidedisturbance of the wound and promotes more rapid healing.

In our co-pending International patent application, WO 2004/037334,apparatus, a wound dressing and a method for aspirating, irrigating andcleansing wounds are described. In very general terms, this inventiondescribes the treatment of a wound by the application of topicalnegative pressure (TNP) therapy for aspirating the wound together withthe further provision of additional fluid for irrigating and/orcleansing the wound, which fluid, comprising both wound exudates andirrigation fluid, is then drawn off by the aspiration means andcirculated through means for separating the beneficial materials thereinfrom deleterious materials. The materials which are beneficial to woundhealing are recirculated through the wound dressing and those materialsdeleterious to wound healing are discarded to a waste collection bag orvessel.

In our co-pending International patent application, WO 2005/04670,apparatus, a wound dressing and a method for cleansing a wound usingaspiration, irrigation and cleansing wounds are described. Again, invery general terms, the invention described in this document utilisessimilar apparatus to that in WO 2004/037334 with regard to theaspiration, irrigation and cleansing of the wound, however, it furtherincludes the important additional step of providing heating means tocontrol the temperature of that beneficial material being returned tothe wound site/dressing so that it is at an optimum temperature, forexample, to have the most efficacious therapeutic effect on the wound.

In our co-pending International patent application, WO 2005/105180,apparatus and a method for the aspiration, irrigation and/or cleansingof wounds are described. Again, in very general terms, this documentdescribes similar apparatus to the two previously mentioned documentshereinabove but with the additional step of providing means for thesupply and application of physiologically active agents to the woundsite/dressing to promote wound healing.

The content of the above references is included herein by reference.

However, the above apparatus and methods are generally only applicableto a patient when hospitalised as the apparatus is complex, needingpeople having specialist knowledge in how to operate and maintain theapparatus, and also relatively heavy and bulky, not being adapted foreasy mobility outside of a hospital environment by a patient, forexample.

Some patients having relatively less severe wounds which do not requirecontinuous hospitalisation, for example, but whom nevertheless wouldbenefit from the prolonged application of TNP therapy, could be treatedat home or at work subject to the availability of an easily portable andmaintainable TNP therapy apparatus.

GB-A-2 307 180 describes a portable TNP therapy unit which may becarried by a patient clipped to belt or harness. It will however beappreciated that from time to time errors may occur during operation ofthe TNP therapy unit. One particular problem which can occur is thatwhen a canister utilised to filter and store waste product becomes fullcorrect operation of the TNP system can be impeded. With current devicestwo pressure sensors are required on each side of a canister to detectsuch an event. A change in measured pressure between the two sensorsimplies a blocked canister filter which further implies a full canister.It will be appreciated that the use of two such sensors is bothexpensive and prone to error and requires complex processing elements todetermine when a canister is full.

SUMMARY

It is an aim of the present invention to at least partly mitigate theabove-mentioned problems.

It is an aim of embodiments of the present invention to provide a methodand apparatus of determining status of a canister of a TNP system. Moreparticularly, but not exclusively, it is an aim of embodiments of thepresent invention to provide a method and apparatus for determining whena canister of a TNP system is full.

It is an aim of embodiments of the present invention to provide anindication of when a canister of a TNP system is full without arequirement for two pressure sensors in the TNP system.

According to a first aspect of the present invention there is provided amethod of determining status in a canister of a topical negativepressure (TNP) system, comprising the steps of:

-   -   monitoring pressure provided by a pump element of the TNP        system;    -   determining at least one characteristic associated with the        monitored pressure; and    -   determining status of at least one parameter associated with a        canister of the TNP system responsive to the determined        characteristics.

The invention is comprised in part of an overall apparatus for theprovision of TNP therapy to a patient in almost any environment. Theapparatus is lightweight, may be mains or battery powered by arechargeable battery pack contained within a device (henceforth, theterm “device” is used to connote a unit which may contain all of thecontrol, power supply, power supply recharging, electronic indicatormeans and means for initiating and sustaining aspiration functions to awound and any further necessary functions of a similar nature). Whenoutside the home, for example, the apparatus may provide for an extendedperiod of operation on battery power and in the home, for example, thedevice may be connected to the mains by a charger unit whilst stillbeing used and operated by the patient.

The overall apparatus of which the present invention is a partcomprises: a dressing covering the wound and sealing at least an openend of an aspiration conduit to a cavity formed over the wound by thedressing; an aspiration tube comprising at least one lumen therethroughleading from the wound dressing to a waste material canister forcollecting and holding wound exudates/waste material prior to disposal;and, a power, control and aspiration initiating and sustaining deviceassociated with the waste canister.

The dressing covering the wound may be any type of dressing normallyemployed with TNP therapy and, in very general terms, may comprise, forexample, a semi-permeable, flexible, self-adhesive drape material, as isknown in the dressings art, to cover the wound and seal with surroundingsound tissue to create a sealed cavity or void over the wound. There mayaptly be a porous barrier and support member in the cavity between thewound bed and the covering material to enable an even vacuumdistribution to be achieved over the area of the wound. The porousbarrier and support member being, for example, a foam or known woundcontact type material resistant to crushing under the levels of vacuumcreated and which permits transfer of wound exudates across the woundarea to the aspiration conduit sealed to the flexible cover drape overthe wound.

The aspiration conduit may be a plain flexible tube, for example, havinga single lumen therethrough and made from a plastics material compatiblewith raw tissue, for example. However, the aspiration conduit may have aplurality of lumens therethrough to achieve specific objectives relatingto the invention. A portion of the tube sited within the sealed cavityover the wound may have a structure to enable continued aspiration andevacuation of wound exudates without becoming constricted or blockedeven at the higher levels of the negative pressure range envisaged.

It is envisaged that the negative pressure range for the apparatusembodying the present invention may be between about −50 mmHg and −200mmHg (note that these pressures are relative to normal ambientatmospheric pressure thus, −200 mmHg would be about 560 mmHg inpractical terms). Aptly, the pressure range may be between about −75mmHg and −150 mmHg. Alternatively a pressure range of upto −75 mmHg,upto −80 mmHg or over −80 mmHg can be used. Also aptly a pressure rangeof below −75 mmHg could be used. Alternatively a pressure range of over−100 mmHg could be used or over −150 mmHg.

The aspiration conduit at its distal end remote from the dressing may beattached to the waste canister at an inlet port or connector. The devicecontaining the means for initiating and sustaining aspiration of thewound/dressing may be situated between the dressing and waste canister,however, in a preferred embodiment of the apparatus embodying thepresent invention, the device may aspirate the wound/dressing via thecanister thus, the waste canister may preferably be sited between thewound/dressing and device.

The aspiration conduit at the waste material canister end may preferablybe bonded to the waste canister to prevent inadvertent detachment whenbeing caught on an obstruction, for example.

The canister may be a plastics material moulding or a composite unitcomprising a plurality of separate mouldings. The canister may aptly betranslucent or transparent in order to visually determine the extent offilling with exudates. However, the canister and device may in someembodiments provide automatic warning of imminent canister fullcondition and may also provide means for cessation of aspiration whenthe canister reaches the full condition.

The canister may be provided with filters to prevent the exhaust ofliquids and odours therefrom and also to prevent the expulsion ofbacteria into the atmosphere. Such filters may comprise a plurality offilters in series. Examples of suitable filters may comprise hydrophobicfilters of 0.2 μm pore size, for example, in respect of sealing thecanister against bacteria expulsion and 1 μm against liquid expulsion.

Aptly, the filters may be sited at an upper portion of the wastecanister in normal use, that is when the apparatus is being used orcarried by a patient the filters are in an upper position and separatedfrom the exudate liquid in the waste canister by gravity. Furthermore,such an orientation keeps the waste canister outlet or exhaust exit portremote from the exudate surface.

Aptly the waste canister may be filled with an absorbent gel such asISOLYSEL (trade mark), for example, as an added safeguard againstleakage of the canister when full and being changed and disposed of.Added advantages of a gel matrix within the exudate storing volume ofthe waste canister are that it prevents excessive movement, such asslopping, of the liquid, minimises bacterial growth and minimisesodours.

The waste canister may also be provided with suitable means to preventleakage thereof both when detached from the device unit and also whenthe aspiration conduit is detached from the wound site/dressing.

The canister may have suitable means to prevent emptying by a user(without tools or damage to the canister) such that a full or otherwiseend-of-life canister may only be disposed of with waste fluid stillcontained.

The device and waste canister may have mutually complementary means forconnecting a device unit to a waste canister whereby the aspirationmeans in the device unit automatically connects to an evacuation port onthe waste canister such that there is a continuous aspiration path fromthe wound site/dressing to an exhaust port on the device.

Aptly, the exhaust port from the fluid path through the apparatus isprovided with filter means to prevent offensive odours from beingejected into the atmosphere.

In general terms the device unit comprises an aspirant pump; means formonitoring pressure applied by the aspirant pump; a flowmeter to monitorfluid flow through the aspirant pump; a control system which controlsthe aspirant pump in response to signals from sensors such as thepressure monitoring means and the flowmeter, for example, and whichcontrol system also controls a power management system with regard to anon-board battery pack and the charging thereof and lastly a userinterface system whereby various functions of the device such aspressure level set point, for example, may be adjusted (includingstopping and starting of the apparatus) by a user. The device unit maycontain all of the above features within a single unified casing.

In view of the fact that the device unit contains the majority of theintrinsic equipment cost therein ideally it will also be able to surviveimpact, tolerate cleaning in order to be reusable by other patients.

In terms of pressure capability the aspiration means may be able toapply a maximum pressure drop of at least −200 mmHg to a woundsite/dressing. The apparatus is capable of maintaining a predeterminednegative pressure even under conditions where there is a small leak ofair into the system and a high exudate flow.

The pressure control system may prevent the minimum pressure achievedfrom exceeding for example −200 mmHg so as not to cause undue patientdiscomfort. The pressure required may be set by the user at a number ofdiscreet levels such as −50, −75, −100, −125, −150, −175 mmHg, forexample, depending upon the needs of the wound in question and theadvice of a clinician. Thus suitable pressure ranges in use may be from−25 to −80 mmHg, or −50 to −76 mmHg, or −50 to −75 mmHg as examples. Thecontrol system may also advantageously be able to maintain the setpressure within a tolerance band of +/−10 mmHg of the set point for 95%of the time the apparatus is operating given that leakage and exudationrates are within expected or normal levels.

Aptly, the control system may trigger alarm means such as a flashinglight, buzzer or any other suitable means when various abnormalconditions apply such as, for example: pressure outside set value by alarge amount due to a gross leak of air into system; duty on theaspiration pump too high due to a relatively smaller leakage of air intothe system; pressure differential between wound site and pump is toohigh due, for example, to a blockage or waste canister full.

The apparatus of the present invention may be provided with a carry caseand suitable support means such as a shoulder strap or harness, forexample. The carry case may be adapted to conform to the shape of theapparatus comprised in the joined together device and waste canister. Inparticular, the carry case may be provided with a bottom opening flap topermit the waste canister to be changed without complete removal of theapparatus from the carry case.

The carry case may be provided with an aperture covered by adisplaceable flap to enable user access to a keypad for varying thetherapy applied by the apparatus.

According to a second aspect of the present invention, there is providedapparatus for determining status in a canister of a topical negativepressure (TNP) system, comprising:

-   -   a canister arranged to collect exudate from an aspirant tube        locatable at a wound site;    -   a pump element arranged to pump air and/or exudate from the tube        through the canister;    -   a pressure sensor for monitoring pressure provided by the pump        element; and    -   a processing unit comprising at least one processing element        that determines at least one characteristic associated with the        monitored pressure and determines status of at least one        parameter associated with the canister responsive to the        determined characteristic.

According to a third aspect of the present invention there is provided amethod of determining the occurrence of a blockage of a canister filterin a topical negative pressure (TNP) system comprising the steps of:

-   -   monitoring pressure provided by a pump element of the TNP        system; and    -   determining if a monitored pressure falls below a pre-determined        threshold value.

According to a fourth aspect of the present invention there is providedapparatus for determining the occurrence of a blockage of a canisterfilter in a topical negative pressure (TNP) system, comprising:

-   -   a canister arranged to collect exudate from an aspirant tube        locatable at a wound site;    -   a filter arranged to filter air in the canister;    -   a pump element arranged to pump air and/or exudate from the tube        through the canister;    -   a pressure sensor arranged to monitor pressure generated by the        pump; and    -   a processing unit comprising at least one processing element        arranged to determine if a monitored pressure falls below a        pre-determined threshold value.

Embodiments of the present invention provide a method and apparatuswhich allows the status of a canister of a topical negative pressure(TNP) system to be determined without the necessity to provide twopressure sensors in the TNP system. By monitoring the magnitude ofpressure ‘pulses’ created by a pump possible leakage or the fact that acanister filter may be full can be detected. Optionally two or moresensors can be used if very prompt detection of errors is desired.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the present invention may be more fully understood,examples will now be described by way of illustration only withreference to the accompanying drawings, of which:

FIG. 1 shows a generalised schematic block diagram showing a generalview of an apparatus and the constituent apparatus features thereof;

FIG. 2 shows a similar generalised schematic block diagram to FIG. 1 andshowing fluid paths therein;

FIG. 3 shows a generalised schematic block diagram similar to FIG. 1 butof a device unit only and showing power paths for the various powerconsuming/producing features of the apparatus;

FIG. 4 shows a similar generalised schematic block diagram to FIG. 3 ofthe device unit and showing control system data paths for controllingthe various functions and components of the apparatus;

FIG. 5 shows a perspective view of an apparatus;

FIG. 6 shows a perspective view of an assembled device unit of theapparatus of FIG. 5;

FIG. 7 shows an exploded view of the device unit of FIG. 6;

FIG. 8 shows a partially sectioned side elevation view through theinterface between a waste canister and device unit of the apparatus;

FIG. 9 shows a cross section through a waste canister of the apparatusof FIGS. 5 to 8;

FIG. 10 illustrates part of a TNP system;

FIG. 11 illustrates how magnitude and/or frequency of pulses can vary;

FIG. 12 illustrates pressure with a blocked canister filter.

DETAILED DESCRIPTION

Referring now to FIGS. 1 to 4 of the drawings and where the same orsimilar features are denoted by common reference numerals.

FIG. 1 shows a generalised schematic view of an apparatus 10 of aportable topical negative pressure (TNP) system. It will be understoodthat embodiments of the present invention are generally applicable touse in such a TNP system. Briefly, negative pressure wound therapyassists in the closure and healing of many forms of “hard to heal”wounds by reducing tissue oedema; encouraging blood flow and granulartissue formation; removing excess exudate and may reduce bacterial load(and, therefore, infection). In addition the therapy allows for lessdisturbance of a wound leading to more rapid healing. The TNP system isdetailed further hereinafter but in summary includes a portable bodyincluding a canister and a device with the device capable of providingan extended period of continuous therapy within at least a one year lifespan. The system is connected to a patient via a length of tubing withan end of the tubing operably secured to a wound dressing on thepatient.

More particularly, as shown in FIG. 1, the apparatus comprises anaspiration conduit 12 operably and an outer surface thereof at one endsealingly attached to a dressing 14. The dressing 14 will not be furtherdescribed here other than to say that it is formed in a known mannerfrom well know materials to those skilled in the dressings art to createa sealed cavity over and around a wound to be treated by TNP therapywith the apparatus of the present invention. The aspiration conduit hasan in-line connector 16 comprising connector portions 18, 20intermediate its length between the dressing 14 and a waste canister 22.The aspiration conduit between the connector portion 20 and the canister22 is denoted by a different reference numeral 24 although the fluidpath through conduit portions 12 and 24 to the waste canister iscontinuous. The connector portions 18, 20 join conduit portions 12, 24in a leak-free but disconnectable manner. The waste canister 22 isprovided with filters 26 which prevent the escape via an exit port 28 ofliquid and bacteria from the waste canister. The filters may comprise a1 μm hydrophobic liquid filter and a 0.2 μm bacteria filter such thatall liquid and bacteria is confined to an interior waste collectingvolume of the waste canister 22. The exit port 28 of the waste canister22 mates with an entry/suction port 30 of a device unit 32 by means ofmutually sealing connector portions 34, 36 which engage and sealtogether automatically when the waste canister 22 is attached to thedevice unit 32, the waste canister 22 and device unit 32 being heldtogether by catch assemblies 38, 40. The device unit 32 comprises anaspirant pump 44, an aspirant pressure monitor 46 and an aspirantflowmeter 48 operably connected together. The aspiration path takes theaspirated fluid which in the case of fluid on the exit side of exit port28 is gaseous through a silencer system 50 and a final filter 52 havingan activated charcoal matrix which ensures that no odours escape withthe gas exhausted from the device 32 via an exhaust port 54. The filter52 material also serves as noise reducing material to enhance the effectof the silencer system 50. The device 32 also contains a battery pack 56to power the apparatus which battery pack also powers the control system60 which controls a user interface system 62 controlled via a keypad(not shown) and the aspiration pump 44 via signals from sensors 46, 48.A power management system 66 is also provided which controls power fromthe battery pack 56, the recharging thereof and the power requirementsof the aspirant pump 44 and other electrically operated components. Anelectrical connector 68 is provided to receive a power input jack 70from a SELV power supply 72 connected to a mains supply 74 when the userof the apparatus or the apparatus itself is adjacent a convenient mainspower socket.

FIG. 2 shows a similar schematic representation to FIG. 1 but shows thefluid paths in more detail. The wound exudate is aspirated from thewound site/dressing 14 via the conduit 12, the two connector portions18, 20 and the conduit 24 into the waste canister 22. The waste canister22 comprises a relatively large volume 80 in the region of 500 ml intowhich exudate from the wound is drawn by the aspiration system at anentry port 82. The fluid 84 drawn into the canister volume 80 is amixture of both air drawn into the dressing 14 via the semi-permeableadhesive sealing drape (not shown) and liquid 86 in the form of woundexudates. The volume 80 within the canister is also at a loweredpressure and the gaseous element 88 of the aspirated fluids is exhaustedfrom the canister volume 80 via the filters 26 and the waste canisterexhaust exit port 28 as bacteria-free gas. From the exit port 28 of thewaste canister to the final exhaust port 54 the fluid is gaseous only.

FIG. 3 shows a schematic diagram showing only the device portion of theapparatus and the power paths in the device of the apparatus embodyingthe present invention. Power is provided mainly by the battery pack 56when the user is outside their home or workplace, for example, however,power may also be provided by an external mains 74 supplied chargingunit 72 which when connected to the device 32 by the socket 68 iscapable of both operating the device and recharging the battery pack 56simultaneously. The power management system 66 is included so as to beable to control power of the TNP system. The TNP system is arechargeable, battery powered system but is capable of being rundirectly from mains electricity as will be described hereinafter morefully with respect to the further figures. If disconnected from themains the battery has enough stored charge for approximately 8 hours ofuse in normal conditions. It will be appreciated that batteries havingother associated life times between recharge can be utilised. Forexample batteries providing less than 8 hours or greater than 8 hourscan be used. When connected to the mains the device will run off themains power and will simultaneously recharge the battery if depletedfrom portable use. The exact rate of battery recharge will depend on theload on the TNP system. For example, if the wound is very large or thereis a significant leak, battery recharge will take longer than if thewound is small and well sealed.

FIG. 4 shows the device 32 part of the apparatus embodying the presentinvention and the data paths employed in the control system for controlof the aspirant pump and other features of the apparatus. A key purposeof the TNP system is to apply negative pressure wound therapy. This isaccomplished via the pressure control system which includes the pump anda pump control system. The pump applies negative pressure; the pressurecontrol system gives feedback on the pressure at the pump head to thecontrol system; the pump control varies the pump speed based on thedifference between the target pressure and the actual pressure at thepump head. In order to improve accuracy of pump speed and hence providesmoother and more accurate application of the negative pressure at awound site, the pump is controlled by an auxiliary control system. Thepump is from time to time allowed to “free-wheel” during its duty cycleby turning off the voltage applied to it. The spinning motor causes a“back electro-motive force” or BEMF to be generated. This BEMF can bemonitored and can be used to provide an accurate measure of pump speed.The speed can thus be adjusted more accurately than can prior art pumpsystems.

According to embodiments of the present invention, actual pressure at awound site is not measured but the difference between a measuredpressure (at the pump) and the wound pressure is minimised by the use oflarge filters and large bore tubes wherever practical. If the pressurecontrol measures that the pressure at the pump head is greater than atarget pressure (closer to atmospheric pressure) for a period of time,the device sends an alarm and displays a message alerting the user to apotential problem such as a leak.

In addition to pressure control a separate flow control system can beprovided. A flow meter may be positioned after the pump and is used todetect when a canister is full or the tube has become blocked. If theflow falls below a certain threshold, the device sounds an alarm anddisplays a message alerting a user to the potential blockage or fullcanister.

Referring now to FIGS. 5 to 9 which show various views and crosssections of a preferred embodiment of apparatus 200 embodying thepresent invention. The preferred embodiment is of generally oval shapein plan and comprises a device unit 202 and a waste canister 204connected together by catch arrangements 206. The device unit 202 has aliquid crystal display (LCD) 208, which gives text based feedback on thewound therapy being applied, and a membrane keypad 210, the LCD beingvisible through the 232, 234, respectively and left-hand and right-handside inserts 236, 238. Inside the casing 230 is a central chassis 240which is fastened to an internal moulded structural member 242 and whichchassis acts as a mounting for the electrical circuitry and componentsand also retains the battery pack 246 and aspiration pump unit 248.Various tubing items 250, 252, 254 connect the pump unit 248 andsuction/entry port 216 to a final gaseous exhaust via a filter 290. FIG.8 shows a partially sectioned side elevation of the apparatus 200, thepartial section being around the junction between the device unit 202and the waste canister 204, a cross section of which is shown at FIG. 9.These views show the rebated edge 218 of the male formation on thedevice unit co-operating with the female portion 220 defined by anupstanding flange 260 around the top face 262 of the waste canister 204.When the waste canister is joined to the device unit, the spigot 214which has an “O” ring seal 264 therearound sealingly engages with acylindrical tube portion 266 formed around an exhaust/exit port 268 inthe waste canister. The spigot 214 of the device is not rigidly fixed tothe device casing but is allowed to “float” or move in its locationfeatures in the casing to permit the spigot 214 and seal 264 to move toform the best seal with the bore of the cylindrical tube portion 266 onconnection of the waste canister to the device unit. The waste canister204 in FIG. 9 is shown in an upright orientation much as it would bewhen worn by a user. Thus, any exudate 270 would be in the bottom of theinternal volume of waste receptacle portion 272. An aspiration conduit274 is permanently affixed to an entry port spigot 278 defining an entryport 280 to receive fluid aspirated from a wound (not shown) via theconduit 274. Filter members 282 comprising a 0.2 μm filter and 284comprising a 1 μm filter are located by a filter retainer moulding 286adjacent a top closure member or bulkhead 288 the filter memberspreventing any liquid or bacteria from being drawn out of the exhaustexit port 268 into the pump and aspiration path through to an exhaustand filter unit 290 which is connected to a casing outlet moulding at291 via an exhaust tube (not shown) in casing side piece 236. The sidepieces 236, 238 are provided with recesses 292 having support pins 294therein to locate a carrying strap (not shown) for use by the patient.The side pieces 230 and canister 204 are also provided with featureswhich prevent the canister and device from exhibiting a mutual “wobble”when connected together. Ribs (not shown) extending between the canistertop closure member 288 and the inner face 300 of the upstanding flange260 locate in grooves 302 in the device sidewalls when canister anddevice are connected. The casing 230 also houses all of the electricalequipment and control and power management features, the functioning ofwhich was described briefly with respect to FIGS. 3 and 4 hereinabove.The side piece 238 is provided with a socket member 298 to receive acharging jack from an external mains powered battery charger (both notshown).

FIG. 10 illustrates schematically a TNP system. A more thoroughdiscussion of many of the parts shown have been made previously withrespect to FIG. 1. It is to be noted that a connecting tube 1000 isillustrated as connecting the dressing 14 to the canister 22 whilst afurther connecting tube 1001 is illustrated connecting the canister tothe aspirant system 32. These tubes are shown for illustrative purposesand it will be appreciated that rather than the tubes connector portions34, 36 can be utilised between the aspirant system and canister and thatlikewise an inline connector 16 may be connected between the dressingand canister.

An aspirant pump 44 used to create the negative pressure of the TNPsystem is a diaphragm pump. This is utilised to move air and create thevacuum in the wound bed. The diaphragm acts as a miniature piston and,hence creates small pulses of pressure as it moves backwards andforwards. These pulses interfere with the flow of air through the systemand their magnitude as measured, for example at the pump inlet, variesaccording to the status of the canister. This relationship isillustrated more clearly in FIG. 11. It will be appreciated that othertypes of pump providing a pulsatile output can be used according toother embodiments of the present invention.

As shown in FIG. 11 during a normal mode of operation the pressurepulses have relatively small magnitude centred around a pre-set pressureP_(set). A maximum value of these pressure pulse readings P_(normal) canthus be utilised to determine when a pump is working efficiently.

Thus by measuring the magnitude of the pressure pulses it is possible todetect whether a canister is blocked. FIG. 11 also illustrates operationwith a canister filter full. Whilst the negative pressure delivered bythe pump remains less than atmospheric pressure P_(atm) the magnitude ofthe pulses is shown as later increased substantially above thepredetermined normal operating pressure P_(normal). It will also beappreciated that minimum pressure values taken at the minimum of thepressure curve or some other common sampling point could be utilised andcompared as a predetermined set value. FIG. 11 thus illustrates howduring a normal mode of operation the flow path provided by the tubing1000, canister 22, tubing 1000 and tubing in the aspirant systemprovides a sufficiently large volume so that pulsatile elements ofpressure variation caused by the diaphragm of the pump are moderated butstill are detectable. When a canister filter 26 becomes full the flowpath volume ‘seen’ by the aspirant pump is much diminished and includesthe volume only of the tubing 1001 and tubing elements in the aspirationsystem. As such the pulsatile elements associated with the pumpingpressure are ‘magnified’.

It will also be appreciated that the frequency of pumping may also varywhen a canister filter becomes full. The frequency can thus likewiseadditionally or optionally be utilised to determine status of at leastone parameter such as fullness or leakiness associated with a canisterof a TNP system.

Rather than initiating an alarm when the canister filter is full, themagnitude or frequency characteristics of the pressure can also becontinually or periodically monitored with a magnitude being used toindicate current status. This can continually provide an indication suchas percentage fullness which may be displayed via a user interface.

It will be appreciated that aptly the pressure is measured close to thelocation where the aspirant pump is provided in a TNP system. This isbecause damping effects caused by the volume of air in the flow path areminimised close to the pump inlet.

FIG. 12 illustrates how embodiments of the present invention can utilisean optional pressure sensor 1002 to monitor pressure at a locationdownstream of a canister filter 26 between the filter and dressing 14.As illustrated in FIG. 12 due to the substantial volume of the flow pathduring normal operation at the pressure sensor 1002 the pulsatileeffects on pressure are muffled somewhat. However as a canister filterfills this results in a blockage in the flow path. The sensor 1002 thusno longer measures any pulse like flow from the pump. When this measuredpressure falls below a predetermined threshold value P_(SET) an alarm inthe form of an audible and/or visual cue can be initiated.

Embodiments of the present invention thus provide a manner in which thestatus of a canister such as a fullness of a filter associated with acanister can be determined by monitoring pressure provided by a pump ofa TNP system. By determining a characteristic such as magnitude orfrequency associated with the monitored pressure the status of at leastone parameter such as fullness or a leak in a flow path associated witha canister can be determined. This can be achieved with only a singlepressure sensor which obviates the need associated with prior knowndevices for two pressure sensors.

Embodiments of the present invention utilise a single pressure sensordownstream of a canister filter between a canister filter and a dressingof a TNP system to determine when a canister filter is full and needsreplacing.

Embodiments of the present invention make use of two pressure sensors.One pressure sensor is located proximate to a pump inlet whilst afurther pressure sensor is located downstream of a canister filter. Thisenable prompt detection of a leak and/or full canister filter.

Throughout the description and claims of this specification, the words“comprise” and “contain” and variations of the words, for example“comprising” and “comprises”, means “including but not limited to”, andis not intended to (and does not) exclude other moieties, additives,components, integers or steps.

Throughout the description and claims of this specification, thesingular encompasses the plural unless the context otherwise requires.In particular, where the indefinite article is used, the specificationis to be understood as contemplating plurality as well as singularity,unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties orgroups described in conjunction with a particular aspect, embodiment orexample of the invention are to be understood to be applicable to anyother aspect, embodiment or example described herein unless incompatibletherewith.

The invention claimed is:
 1. A method of operating a topical negativepressure (TNP) system comprising a negative pressure source, a pressuresensor, a canister, and a controller, the method comprising: aspirating,by the negative pressure source, fluid from a wound; collecting, by thecanister, at least some fluid removed from the wound; monitoring, by thepressure sensor, pressure pulses generated by the negative pressuresource; comparing, by the controller, a magnitude of the pressure pulsesto a magnitude threshold, wherein the magnitude of the pressure pulsesincreases as a level of fluid in the canister increases; by thecontroller, determining, before a canister filter associated with thecanister becomes blocked, that the level of fluid in the canister isabove a canister empty level and below a canister full level based onthe comparison of the magnitude of the pressure pulses to the magnitudethreshold; and outputting, by the controller, to a user interface anindication for presentation to a user, the indication denoting that thelevel of fluid in the canister is above the canister empty level andbelow the canister full level.
 2. The method of claim 1, furthercomprising: comparing a frequency of the pressure pulses to a frequencythreshold; and determining the level of fluid in the canister based onthe comparison of the frequency of the pressure pulses to the frequencythreshold.
 3. The method of claim 2, wherein said comparing thefrequency of the pressure pulses comprises comparing the frequency ofthe pressure pulses to the frequency threshold over a period of time. 4.The method of claim 1, wherein said comparing comprises comparing themagnitude of the pressure pulses to the magnitude threshold over aperiod of time.
 5. The method of claim 1, wherein said monitoringcomprises monitoring the pressure pulses at an inlet of the negativepressure source.
 6. The method of claim 1, further comprising:comparing, by the controller, the magnitude of the pressure pulses toanother magnitude threshold; by the controller, determining that thelevel of fluid in the canister is at the canister full level based onthe comparison of the magnitude of the pressure pulses to the anothermagnitude threshold; and outputting, by the controller, to the userinterface another indication for presentation to the user, the anotherindication denoting that the level of fluid in the canister is at thecanister full level.
 7. The method of claim 1, wherein that the level offluid in the canister is above the canister empty level and below thecanister full level is determined using pressure data from no pressuresensor other than the pressure sensor.
 8. The method of claim 1, furthercomprising audibly presenting the indication with the user interface tothe user.
 9. The method of claim 1, further comprising visuallypresenting the indication with the user interface to the user.
 10. Atopical negative pressure (TNP) apparatus comprising: a negativepressure source configured to aspirate fluid from a wound; a canisterconfigured to collect at least some fluid removed from the wound; acanister filter associated with the canister; a pressure sensorconfigured to monitor pressure pulses generated by the negative pressuresource; and a controller configured to: compare a magnitude of thepressure pulses to a magnitude threshold, wherein the magnitude of thepressure pulses increases as a level of fluid in the canister increases;and determine, before the canister filter becomes blocked, whether thelevel of fluid in the canister is above a canister empty level and belowa canister full level based on the comparison of the magnitude of thepressure pulses to the magnitude threshold.
 11. The TNP apparatus ofclaim 10, wherein the controller is further configured to: compare afrequency of the pressure pulses to a frequency threshold; and determinethe level of fluid in the canister based on the comparison of thefrequency of the pressure pulses to the frequency threshold.
 12. The TNPapparatus of claim 11, wherein the controller is configured to comparethe frequency of the pressure pulses to the frequency threshold over aperiod of time.
 13. The TNP apparatus of claim 10, wherein thecontroller is configured to compare the magnitude of the pressure pulsesto the magnitude threshold over a period of time.
 14. The TNP apparatusof claim 10, wherein the pressure sensor is configured to monitor thepressure pulses at an inlet of the negative pressure source.
 15. The TNPapparatus of claim 10, further comprising a user interface configured toindicate that the level of fluid in the canister is above the canisterempty level and below the canister full level.
 16. The TNP apparatus ofclaim 15, wherein the user interface is configured to audibly indicatethat the level of fluid in the canister is above the canister emptylevel and below the canister full level.
 17. The TNP apparatus of claim15, wherein the user interface is configured to visually indicate thatthe level of fluid in the canister is above the canister empty level andbelow the canister full level.
 18. The TNP apparatus of claim 15,wherein the user interface is configured to indicate with a percentagefullness indicator that the level of fluid in the canister is above thecanister empty level and below the canister full level.
 19. The TNPapparatus of claim 10, wherein the controller is further configured to:compare the magnitude of the pressure pulses to another magnitudethreshold; and determine whether the level of fluid in the canister isat the canister full level based on the comparison of the magnitude ofthe pressure pulses to the another magnitude threshold.
 20. The TNPapparatus of claim 10, wherein the controller is configured todetermine, before the canister filter becomes blocked, whether the levelof fluid in the canister is above the canister empty level and below thecanister full level with pressure data from no pressure sensor otherthan the pressure sensor.